1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus including a recording head for ejecting liquid droplets and a head tank for supplying liquid to the recording head.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, inkjet recording apparatuses are known that use a recording head (liquid ejection head or droplet ejection head) for ejecting droplets of ink or other liquid.
Such a liquid-ejection-type image forming apparatus may have a head tank (also referred to as sub tank or buffer tank) to supply ink to the recording head, and the head tank has a function (mechanism) to create a negative pressure to prevent ink from seeping or dropping from nozzles of the recording head.
For example, JP-4190001-B1 (JP-2005-059274-A) proposes an image forming apparatus including a head tank (sub tank), a main tank, a full state detector, and a controller. The head tank includes a deformable film member forming at least one face of the head tank, an elastic member to urge the film member outward, a negative pressure generation unit to generate a negative pressure in the head tank by expansion and contraction in response to liquid supply and discharge, a displacement member contacting an outer face of the film member and displaceable with deformation of the film member, and an air release unit to open an interior of the head tank to the atmosphere. The main tank supplies liquid to the head tank. The full state detector detects the position of the displacement member of the head tank. The controller opens the air release unit to release the interior of the head tank to the atmosphere and supplies liquid from the main tank to the head tank with the air release unit opened. Then, the controller closes the air release unit and contracts the negative pressure generation unit by discharging a part of liquid from the head tank to generate a negative pressure in the head tank. When the negative pressure is generated in the head tank, the controller stores a current position of the displacement member (also referred to as feeler) as a normal fill feeler position. During liquid supply from the main tank to the head tank with the air release unit closed, the controller stops the liquid supply when the displacement member arrives at the normal fill feeler position stored.
However, for the configuration of JP-4190001-B1 (JP-2005-059274-A) in which, with the air release unit closed, liquid is filled (supplied from the main tank to the head tank) till the displacement member arrives at the normal fill feeler position stored, if the normal fill feeler position is beyond a displaceable range of the displacement member, the displacement member may not move with consumption of liquid in the head tank, thus hampering proper liquid filling (i.e., the controller determines that liquid filling has been completed and does not perform liquid filling).
When liquid is fully filled to the head tank with the air release unit opened, the controller stores a current position of the displacement member as a liquid full position (liquid full feeler position). However, the liquid full position of the displacement member may vary with the number of operation of the film member or ambient environment. As a result, even if the discharge amount of liquid discharged from the head tank to create a negative pressure in the head tank is constant, the normal fill feeler position may be beyond the displaceable range of the displacement member.
In such a case, since liquid in the head tank is consumed without the replenishment (filling) of liquid, liquid may run out in the head tank or an excessive negative pressure may be created in the head tank, thus causing ejection failure.